That seems rather low for a 550W PSU IMO. Should be around 40A. And I highly doubt it puts out what the label says since I've never heard of such a brand.
To be honest, I think your CPU is going to seriously limit any video card you buy. Pentium D's are really slow, AMD X2's are faster with lower clocks.
I'd look at upgrading the CPU, PSU and GPU but that's going to be costly. Unfortunately, your motherboard doesn't support 1066FSB either so you could only get a E2x00 CPU at the most. To me, looks like you should save up (maybe hope that the economy gets better too) and do a whole system overhaul.
I totally agree with this statement, it wouldn't be prudent to steer you towards a new GPU with your current CPU and PSU!! Hey guys, look at the games he wants to play and at that Res??....no way....you need to consider a "new build"....there are a ton of folks on here whom can steer you towards a very good "budget" build or a an insane high end build!!
Introducing me n my OCD to Watercooling, is like taking an Alcoholic to an "all you can drink" Beach Bar in Mexico